Gas bags in restraint systems are inflated by gas from a stored supply of inert gas under high pressure, by gas generated locally from a gas generating pyrotechnic material such as a sodium azide containing composition or by gas derived from the so-called hybrid or augmented gas system comprising stored compressed gas and also a gas and heat generating pyrotechnic material. In the hybrid system the generated hot gas mixes with and expands or increases the pressure of the stored compressed gas. Consequently, the amount of gas required to be stored is substantially less than in systems relying only on stored gas and the gas storage pressure and/or the overall size of the inflator is generally substantially reduced. Moreover, although the hybrid system is bulkier than the system using only a pyrotechnic material, it is often preferred as it produces less solids and noxious gases in the gas mixture supplied to the bag.
Typical hybrid inflators are described in United Kingdom patent specification no. 1408530 and in U.S. Pat. Nos. 4,131,300; 5,022,674; 5,031,932 and 5,033,772.
In hybrid inflators used hitherto the pyrotechnic hot gas generating composition is contained in a separate pressure vessel which is usually placed within the pressure vessel containing the stored pressurised gas and both containers are arranged to deliver gas to a mixing zone before the mixed inflation gases pass into the bag. This arrangement of apparatus is complicated, requires accurately constructed flow control devices and is expensive. A further disadvantage of a hybrid inflator is the relatively large particle size of the solid products from the pyrotechnic composition which products can cause blockage of gas delivery passages and are relatively inefficient in transferring heat to the stored gas.